A SPAD is based on a p-n junction device biased beyond its breakdown region. The high reverse bias voltage generates a sufficient magnitude of electric field such that a single charge carrier introduced into the depletion layer of the device can cause a self-sustaining avalanche via impact ionization. The avalanche is quenched, either actively or passively to allow the device to be “reset” to detect further photons. The initiating charge carrier can be photo-electrically generated by way of a single incident photon striking the high field region. It is this feature which gives rise to the name “Single Photon Avalanche Diode”. This single photon detection mode of operation is often referred to as “Geiger Mode”.
Single photon counting devices output response to incident light intensity becomes significantly non-linear as intensity increases. This may be problematic for many applications, such as ambient light sensing and ranging. It may be desirable to provide a SPAD and/or SPAD array with a better linear output response to illumination levels.